This invention relates to synthetic lubricants which are a suitably inhibited blend of (1) an ester of a monohydric alcohol having 4 to 18 carbon atoms with one or more aromatic or alkane dicarboxylic acids and (2) one or more polyether polyols.
Reciprocating air compressors having air cushioned valves are well known in the art.
It is well known to use hydrocarbon lubricating oils to lubricate the pistons and piston rings of the foregoing air compressors and lubricate the bearings. Due to the high temperature and pressure of the air, it has been found that these hydrocarbon oils break down, leave deposits, and prevent the valves from operating correctly in a relatively short time. This requires manual repairs to clean the valves.
It is known that synthetic esters made from dicarboxylic acids have been used to produce long lasting compressor fluids, such as Anderol 495 sold by Nuodex for rotary screw air compressors. The major component of Anderol 495 is believed to be a dialkyl adipate. However, Anderol 495 does not have sufficient high temperature viscosity for suitable lubrication of the pistons and cylinders of reciprocating air compressors.
It is also known from U.S. Pat. No. 4,302,343 that rotary screw air compressors can be lubricated with a blend of polyhydric alcohol esters and polyether polyols. However, these lubricants are relatively expensive and leave deposits on the valves of a reciprocating air compressor.
Anderol 500 (a dialkyl phthalate composition) is known to be useful in reciprocating air compressors. However, this synthetic ester has the disadvantage of having a high viscosity during start up at low temperatures.
The U.S. Pat. No. 4,072,619 polyesteralkylene glycol compositions are disclosed wherein phenothiazine is incorporated into the alkylene glycols. However, these compositions have been found to degrade in a relatively short time i.e. 1000 hours.
Synthetic lubricants comprising a major amount of a polyester and a minor amount of a monocapped polyglycol are known from British Pat. Nos. ,933,721; 986,066; and 1,162,818, however all these compositions are disclosed to be only useful in aircraft gas turbines where a different viscosity range is needed.